Changes

bts: limit stderr logging to NOTICE to avoid long write() to ext4 fs

stderr being redirected to a file in the ext4 filesystem sometimes ends
up in write() syscall taking >500ms, which means the entire osmo-bts
process stalls during that time and we miss clock updates up to a
threshold which makes osmo-bts process exit with an error.

Let's decrease logging verbosity to NOTICE for now with the aim to not
run into those stalls during normal operation, while we can re-eanble
this manually when debugging the write() issue.

Related: OS#6794
Change-Id: I74c4abf7571d2a0f9ee22402a949dbde02896d7d